Are you sure Gentoo as not overwritten grub with its own, so what you are viewing is the gentoo grub menu?
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enzotibJul 25 '11 at 12:04

No, Gentoo is in another hard-disk and I haven't install grub there. Moreover, if Gentoo grub overwrites the Ubuntu grub: why it shows the Ubuntu entries but no the Gentoo entry?
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SimónJul 25 '11 at 12:10